In general, imaging lenses installed in imaging devices, such as photographic film cameras, digital cameras, and video cameras include a stop that blocks incident light and that is either fixed or variably positioned. Such a stop may be an aperture stop that controls the amount of light being transmitted by the imaging lens at any instant or a field stop that defines, for example, the angle of view (field angle) of the imaging lens. An aperture stop may be termed a brightness stop and may be variable as an iris diaphragm. The aperture stop controls the diameter of the light beam at one position along the optical axis of the imaging lens and may affect resolution, contrast, and other features of an image formed by the imaging lens. On the other hand, a field stop or field diaphragm functions to limit the field of view or field angle, which is a factor in determining imaging performance, and is usually designed to form a sharp edge to the field of view with a desired size and shape of the image. Besides aperture stops and field stops, other types of stops are intended to prevent or minimize flare and ghosting. In general, these may be referred to as flare cutting stops or by similar language. Hereinafter, any such structures generally that have the function of controlling the light passing through a lens system will be referred to as a “stop.”
Ordinarily stops, especially aperture stops, are constructed of mechanical components and are a separate component from any lens. A field stop and a flare cutting stop are also often constructed of masking members that are separate from lenses. On the other hand, Japanese Laid-Open Utility Model Application H06-64218 and Japanese Patent No. 2507166 describe methods of providing flare cutting stops without separately installing any masking member. In particular, Japanese Laid-Open Utility Model Application H06-64218 describes an objective lens that forms a light blocking pattern that is used as a flare cutting stop by printing or coating the pattern on a lens surface, and Japanese Patent No. 2507166 describes a projection television equipped with a projection lens that forms a light blocking pattern that is used as a flare cutting stop by printing the pattern on a lens surface.
Because imaging devices, such as those mentioned above, have been made much smaller in recent years, miniaturization of their lens systems in particular has progressed by reducing their total lengths and outer diameters. Especially, imaging lenses of modular cameras for inputting images to portable phones and similar devices that have rapidly become popular, are becoming extremely compact. In imaging lenses of such portable cameras, because the diameters of the incident light beams are extremely small, the thicknesses of the stop members become large relative to the diameters of the incident light beams, in contrast to most imaging lenses, and thus flare and ghosting occur more easily due to reflection and other factors at the surfaces and edges of the member that defines the stop and the aperture of the stop. Especially for recent highly compact imaging lenses, the thickness of mechanical members that are used, for example, as aperture stops cannot be ignored in miniaturizing the lenses.
On the other hand, aperture stops in imaging lenses of portable cameras and similar devices are often made to be fixed stops in order to simplify the construction. It is considered preferable to use fixed stops in general that enable miniaturization and prevent flare by using a light blocking pattern, such as those described in the previously mentioned Japanese Laid-Open Utility Model Application H06-64218 and Japanese Patent No. 2507166, instead of using mechanical stop members. In this case, because the lens surface is directly coated with light blocking paint or similar material, and a light blocking pattern which operates as a stop is formed by coating the light blocking material without requiring the use of separate stop members, the occurrence of flare and ghosting can be prevented. Also, not using a separate stop member assists in achieving miniaturization.
However, imaging lenses of portable cameras have progressed even further in terms of miniaturization. Hence, in directly printing or otherwise adhering a light blocking paint on a lens surface, it becomes difficult to precisely form a light blocking pattern, which may present manufacturing problems. Note that in the lenses described in Japanese Laid-Open Utility Model Application H06-64218 and Japanese Patent No. 2507166, lens systems such as single-lens reflex cameras and projection lenses that are relatively large compared with portable cameras are generally considered, and the problems specific to compact imaging lenses are not specifically considered.